Abstract: A system of adjustable interlockable panels for assembling a modular unit is provided, which comprises a plurality of rectangular, adjustable interlockable panels. Each panel has four edges: a first edge comprises a male sliding interlock, and a third, opposite edge comprises a female sliding interlock; a second edge comprises one or more adjustment slots; and an opposite, fourth edge, is a straight edge. The adjustment slots in the second edge are: parallel to the first and third edges; of a sufficient width to tightly accommodate any of the other panels in the system; and each slot extends between about ¼ and ¾ of the way into the panel. The male sliding interlock on a first panel is complementary to the female sliding interlock on a second panel in the system.